Cytotoxic cells express specific receptors on their surfaces by which they distinguish altered or foreign cells from normal autologous cells. These receptors form multiple links to structures on target cell surfaces, leading to stable conjugates between cytotoxic and target cells. Each cytotoxic cell then delivers a "lethal hit" to its conjugate target cell and detaches from it, leaving a dying target cell and a cytotoxic cell which is free to locate and destroy another target. (Segal, D. M. et al., Cancer Invest. 6(1): 83-92 (1988); Segal, D. M. et al., Mol. Immunol. 25: 1099-1103 (1988)).
Recently, a method has been developed by which the natural recognition system of cytotoxic cells can be artificially manipulated, giving rise to cytotoxic cells of any desired specificity, including specificity against tumor cells (Segal, D. M., et al., U.S. Pat. No. 4,676,980; Karpovsky, B., et al., J. Exp. Med., 160: 1686-1701 (1984); Perez, P., et al., Nature, 316: 354-356 (1985)). The method for retargeting cytotoxic cells employs crosslinked heterobispecific antibodies, in which one antibody is directed against the receptor on the cytotoxic cell which is involved in lysis, while the second antibody is directed against a target cell structure, for example, a tumor antigen. By linking the relevant receptor on the cytotoxic cell directly to the target cell, the crosslinked heterobispecific antibodies promote the formation of effector: target conjugates and signal the cytotoxic cell to deliver a lethal hit. Antibody heteroaggregates can be produced by chemical crosslinking, or by fusing two hybridoma cells. (Segal, D. M., et al., in: Biological Therapy of Cancer Updates Vol. 2, V. T. DeVita, S. Hellman, and S. A. Rosenberg, eds. J. B. Lippincott Co., Philadelphia pp. 1-12 (1992)).
In recent years a great deal of interest has been focused on redirecting cytotoxic cells to kill unwanted neoplastic or virally infected cells. A common way of doing this is to use a bispecific antibody with dual specificity for an antigen on the target cell and a triggering molecule on the effector cell (such as CD3 on T cells). Such bispecific antibodies are being used in a number of clinical trials to target T cells against tumor. (Segal, D. M. and Wunderlich, J. R., Cancer Investigation 6: 83-92 (1988)).
The concept of retargeted effector cells for treatment of pathological conditions, such as cancer, offers some advantages over conventional, non-targeted immunotherapy. However, immune selection of targeted cells over normal cells is still problematic. Increased selectivity may be accomplished by combining forms of therapy, such as radiation and/or chemotherapy in conjunction with immunotherapy. However, these supplemental therapies are often accompanied by serious side effects. Moreover, to reach the targeted cancer cells, these large crosslinked antibodies, must penetrate solid tumor tissue sufficiently to bind to the targeted tumor cell.
Additionally, host immune responses to xenoantibodies (i.e., antibodies produced in species other than the host undergoing treatment) have been observed in clinical trials. These responses could destroy the antitumor specificity of retargeted effector cells. Furthermore, clearance of unbound crosslinked antibodies of this size, as well as clearance of the antibodies following binding, is also a problem.
Finally, retargeted effector cells may lose their artificially acquired tumor receptors (the heterobispecific antibodies) by interaction with tumor cells, by effector cell division, by endocytosis, by proteolytic extracellular enzymes, or by natural shedding. Antitumor activity in the host can be maintained by repeated treatments with effector cells and targeting antibodies. However, it is expensive and time consuming to produce large quantities of heterobispecific antibodies with the specificity necessary to interact with the intended target such as cell surface tumor antigens. Thus, it would be advantageous to be able to produce large amounts of clinical grade bispecific antibodies, for use with many different tumor antigens or cell surface markers for repeated treatment.